Light-diffusing polymers are "semi-transparent" or translucent to visible light, that is, they scatter transmitted light, preferably without significantly reducing its intensity. They may be clear or colored, and may be incorporated into clear or colored thermoplastic or thermoset polymer matrices to form light-scattering thermoplastic or thermoset polymer compositions which may be formed into sheets or films of various thicknesses, or into more complex shapes. Such compositions find use as light diffusers in, for example, luminaires, rear projection screens for television or film viewing, decoration, illuminated signs, especially back-lit translucent signs, skylight, lighting fixtures, especially for fluorescent or incandescent lighting, greenhouse glazing, light boxes, drafting tables, automotive sunroofs, artistic applications, such as visual display-case components, antiglow screens for CRT units, twin-wall glazing, covers for automotive lights and many related applications.
Early light diffusers were made from glass, typically by surface treatment such as grinding or etching. Polymeric light diffusers initially used similar processes, or incorporated finely ground inorganic particles, such as titanium dioxide, barium sulfate and silica, into the polymer. Such particles were difficult to disperse evenly, they reduced the light transmission of the polymer unacceptably, and, because they were usually hard, irregular, ground particles, they tended to abrade the processing equipment.
Glass microspheres solved some of the problems associated with other inorganic particles; they were less abrasive because they were spheres, and they allowed greater light transmission, and they shared a tendency with the other inorganic light diffusers to reduce the physical strength of the polymer.
Light-diffusing polymer particles have also been used to introduce light-diffusing properties into transparent matrix polymers. Land, U.S. Pat. No. 4,165,153, teaches incorporating polymer particles having a refractive index preferably at least 0.1 units below that of the matrix polymer. Lang, U.S. Pat. No. 3,992,486, teaches the use of opaque, porous, crosslinked polymer particles smaller than about 30 micrometers (.mu.m), formed by endopolymerization and having a refractive index preferably above that of the matrix polymer, as an opacifier, flatting agent and pigment as well as a light diffusing agent. Krieg et al., U.S. Pat. No. 3,883,617, teaches precipitating styrene particles within a monomer system during polymerization, and subsequently thermally crosslinking these particles to form light diffusers. As only styrene is taught, no flexibility exists in selecting the refractive index of the particles with respect to the matrix polymer. Kishida et al., in Japanese Patent Publication No. 59-38252, teach suspension-polymerized particles of 10 to 500 .mu.m diameter as light diffusing particles.
Japanese Patent Publication No. 64-10515 discloses a method for producing a translucent resin comprising mixing fine particles of a bridging polymer in a transparent resin of a base material, said bridging polymer particles having an average diameter of 0.5 to 5 .mu.m and differing in refractive index of the matrix by 0.05 to 0.3 units. However, the grow-out process taught therein for making the particles suggests that an appreciable quantity of small particles are produced. The reference requires a bridging monomer which is a crosslinking monomer.
Japanese Kokai 62-84156 describes a grow-out process involving polymerization of mixtures of monofunctional and polyfunctional monomers, followed by a staging with a very high concentration of polyfunctional monomers, with oil-soluble initiators onto a pre-formed seed to form fine granules with a granular diameter of 1-30 .mu.m, preferably 1-20 .mu.m, and standard deviation of below 1 .mu.m, preferably below 0.5 .mu.m. The Japanese document does not disclose or suggest the use of such particles in thermoplastic or thermoset compositions.
Matte-surface or low-gloss polymers are thermoplastic or thermoset materials which scatter light broadly from the surface instead of having a glossy surface with high reflectance. They may be clear, opaque, or colored, and may be formed into sheets or films of various thicknesses or more complex articles.
Surface alteration of thermoplastics to reduce gloss has been carried out commercially for some years. Manufacturing techniques employed have been the use of specially roughened extruded or calendering rolls, extrusion under conditions producing melt fracture such as is taught for acrylic film in U.S. Pat. No. 3,415,796, and the use of small-particle inorganic fillers, such as silica. The first requires special processing equipment; the second requires materials of low melt flow, placing a load on the extruder, and it is difficult to avoid a patterned or "sharkskin" appearance from excessive melt fracture; the third causes wear on thermoplastic processing machinery, and often detracts from the physical properties of the resultant plastic.
Polymeric modifiers for surface alteration are known to the art. Acrylonitrile-butadiene-styrene (ABS) resins have been manufactured in which one component of the resin is a particulate ABS polymer crosslinked to such an extent that it maintains its shape even during the extrusion process, producing surface-alteration. Similar technology is known for poly(vinyl chloride) (Japanese Kokai 83-33426), but the technology utilizes beads of crosslinked PVC similar in size to the uncrosslinked resin. The height of the surface protrusions is taught to be 1-45 .mu.m.
Another useful technology is that taught by Lang in U.S. Pat. Nos. 3,992,486 and 4,000,216, wherein an endopolymer is prepared by polymerizing in bulk or in suspension a monomer mixture containing a crosslinking or graftlinking monomer in the presence of a preformed polymer dissolved therein under conditions where the new polymer is formed as particles of rather broad particle size distribution but in the range of at least about 1 to at least about 30 .mu.m dispersed in a continuous phase of the preformed polymer. These particles dispersed in the preformed polymer are then processed with a suitable polymeric matrix to achieve a desired flatted, matte-surfaced, or low gloss plastic.
Each of the above particles suffers from one or more of the following problems: inflexibility of refractive-index choice, hardness of the particle resulting in poorer impact strength, poor dispersion in the matrix polymer with accompanying degradation of matrix-polymer physical strength, and degradation of light transmission in the matrix polymer. It is an object of the present invention to provide a polymeric additive for thermoplastic or thermoset polymers which reduces one or more of the above problems.